Production of clad metal articles

ABSTRACT

Production of clad stock, e.g., bar, tubing, pipe, etc., wherein the cladding is prepared by pressing metal powders to desired form, the green form is sintered and coextruded hot with the core or basis material. Highly corrosion resistant cladding metals can be applied to basis materials of iron-group metals including common structural materials and high recovery of the cladding material is obtained. Externally or internally clad tubes have a cladding of 50 percent nickel-50 percent chromium alloy on substrates of steels, nickel alloys, including nickel-chromium alloys, etc., can readily be produced for use in applications requiring resistance to severe corrosive conditions at various temperatures.

United States Patent [191 Manilla et al.

[ Aug. 21, 1973 [54] PRODUCTION OF CLAD METAL ARTICLES 2,320,801 6/1943Simons 75/208 2, 7, 7 W51 Charles M-nma; Harold 3336,33 351322 52121..721383 Olen 3,531,848 10/1970 Gripshover et al. 75/208 Gothard, all ofHuntington, W. Va.

[73] Assignee: The International Nickel Company, PrimaryExaminer-Benjamin R. Padgett Inc., New York, N.Y. Assistant Examiner-B.Hunt [22] Filed: Aug. 25 1970 Attorney-Maurice L. Pinel [21] App]. No.:66,788 [57] ABSTRACT Related U.S- Applica i Data Production of cladstock, e.g., bar, tubing, pipe, etc., [63] Continuation-impart of Ser.No. 630,987, Apnl 14, h i the cladding is prepared by pressing meta]1967 powders to desired form, the green form is sintered and coextrudedhot with the core or basis material. Highly [52] US. Cl 75/208 R,75/214, 75/226 Corrosion resistant cladding metals can be applied to[51] Int. Cl. B22f 7/04, B22f 7/02 basis materials ofiromgroup metalsincluding common of Search 214, structural materials and recovery f hcladding material is obtained. Externally or internally clad tubes [5Reierences and have a cladding of 50 percent nickel-50 percent chro-UNITED STATES PATENTS mium alloy on substrates of steels, nickel alloys,includ- 3,361,562 1/1968 Ulrich et a1 75/214 ing nickel-chromium a y c,an ead ly be p o- 2,847,708 8/1958 Hamjian et a1. 75/214 duced for usein applications requiring resistance to se- 2.148.040 1939 Schwarzkopf75/221 vere corrosive conditions at various temperatures. 2,947,0808/1960 Kates et al. 75/208 3,271,849 9/1966 Price 29/1912 4 Claims, 6Drawing Figures r 1 4 I a \i 5% s: x \z a 6 \E l xi z PRODUCTION OF CLADMETAL ARTICLES The present application is a continuation-in-part of ourprior co-pending U.S. application Ser. No. 630,987 filed Apr. 14, 1967now U.S. Pat. No. 3,652,235.

The present invention is directed to the production of composite billetsand to composite metal products produced therefrom and, moreparticularly, to the production of such products wherein metal powdersare employed as a material source.

The art demonstrates a long history in relation to the production ofcomposite products. For example, hotdipped coatings of tin, zinc andlead are provided industrially on steel products in substantial tonnageseach year. Other metal coating and/or cladding methods includeelectrodeposition, metal spray, cementation, wherein metals such aszirconium, chromium, aluminum and silicon are applied to steel; claddingprocesses, wherein a metal such as copper may be cast about or upon asteel core to form a composite ingot which is then worked down; weldingprocesses, wherein corrosion resistant metal coatings such as stainlesssteels, nickel-chromium alloys and other alloys are applied to basematerial, such as carbon steel, hot rolling of sandwich-type slabs, etc.Even paintcoated metals may be considered as composite products.

The commercial acceptance of composite products has been great. One ofthe reasons for such acceptance of composite products is the fact thatmaterials such as steels of various kinds which are strong and low inprice are not sufficiently resistant to destructive media to provide anacceptable useful life in service unless a more corrosion resistantcoating is applied to exposed faces thereof. Coated and/or cladded metalarticles provide economic advantages in that materials which arerelatively cheap and quite strong can be rendered satisfactory for usein environments which would be destructive thereto by applying to thesurface thereof a coating or cladding of more corrosion resistant metal.In general, the coatings and/or claddings are made of materials whichare more expensive than the basis material and the coating and/orcladding procedure enables the production of a relatively inexpensivecomposite article which will, nevertheless, have an acceptable servicelife in a medium which would be destructive to the basis metal.Generally, coatings and/or claddings are relatively thinner than thebasis metal, thereby providing economy in the use of the generally morecostly cladding and/or coating metal.

With advances in technology, the demand for improved service life ofmetallic materials in destructive media at room temperature and atelevated temperatures has increased. For example, in superheater tubesemployed in boilers which are fired by coal or oil, it is found thathighly corrosive conditions are generated from the products ofcombustion. Thus, many cheap fuels, such as residual oil, containvanadium and sulfur and the combustion products of the fuel areextremely corrosive to ferrous and non-ferrous metals with the resultsthat superheater tubes have a very short life. Replacement of the tubesrequires shut down of expensive equipment with resulting high cost. Thisproblem could be solved if a cladding resistant to extremely corrosiveconditions could be applied to a cheaper alloy such as a stainlesssteel, nickel-chromium alloy, etc.

It is known that nickel-chromium alloys containing, for example, 40 or50 percent or more chromium and the balance essentially nickel, offerexceptional resistance to hot atmospheres containing vanadium and sulfurcompounds. However, large ingots made of such alloys are extremelydifficult to hot work. Another constantly occurring demand in connectionwith clad metals is that they be provided in commercial forms at as lowa price as possible.

One method of producing a clad tube, for example, is to insert a billetof a cheaper metal, for example, carbon steel, stainless steel or thelike, within a tube of a more corrosion resistant metal, heat theassembly and hot work the same so as to provide a cladded tube. However,such processes involve two expensive hot tube-forming operations andcomplex auxiliary operations replete with substantial scrap losses andaccordingly are not economically attractive.

It is known to produce cladded metal structures using powder metallurgytechniques. Generally, such techniques have been employed in theproduction of hearing elements in which powdered metals, such as lead,copper, or aluminum are sintered and metallurgically bonded to a metalsubstrate, such as steel and the like, using a combination of sinteringand rolling steps to achieve densification. Such metals are generallyeasy to work with and do not require high processing temperatures.

However, corrosion-resistant chromium-containing alloys have not beengenerally amenable to such treatments. Such powder materials requirehigher working temperatures and a great deal of care must be taken toavoid oxidation which tends to interefere with sintering and withobtaining the desired metallurgical bond with the metal substrate.

Moreover, the characteristics of the cladding material may not always becompatible with the characteristics of the substrate or basis metalusing conventional metal working techniques, whereby cracks may occur inthe coating and render it substantially worthless as a protectivecoating.

in producing a self-supporting layer by pressing a powder mixture in adie prior to application to a metal substrate, it could not always becertain that a crackfree layer could be obtained because of thedifficulty in achieving uniform density throughout the green layer bypressing. In addition, because the layer to be bonded was necessarilyporous during the initial stages of hot working, it was not uncommon forthe layer to rupture during consolidation while in contact with themetal substrate.

As far as we are aware, it has not been possible, prior to ourinvention, to produce economically a cladded metal product in which theclad is a high chromium alloy of the type containing, for example,approximately 50 percent chromium and approximately 50 percent nickel.

A demand accordingly exists in the art not only to provide claddedmetals such as tubing, rods and the like at as low a price as possiblebut also to provide such products with metallurgically bonded claddingsmade of highly corrosion resistant alloys which are generally difficultto hot work from the usual ingot stage.

We have now discovered a method whereby composite products can beproduced in a simple and relatively inexpensive manner with minimizationof scrap losses in relation to the cladding material and which makespossible the production of claddings made of difficultly hot workablealloys.

It is an object of the present invention to provide by powder metallurgymeans an improved process for making cladded metal articles.

It is another object of the present invention to provide claddings madeof highly corrosion resistant metal upon basis metal having poorercorrosion resistance by a powder metallurgy method.

It is a further object of the invention to provide, by powdermetallurgy, metallic composite tubes having an excellent metallurgicalbond between the cladding metal and the basis metal.

A still further object is to provide a composite billet for producing acladded metal product, in which the clad-forming element is aself-sustaining layer of compressed metal powder.

Other objects and advantages of the invention will become apparent fromthe following description taken in conjunction with the accompanyingdrawing in which:

FIG. l.-is a schematic representation ofa mold assembly for molding atube from metal powder in accor dance with the invention;

FIG. 2 is a reproduction of a photograph taken at 0.167 diameter showingthe surface of clad tubes produced in accordance with the invention;

FIG. 3 is a reproduction of a photograph taken at 1.85 diametersdepicting the etched surface of a cross section of a clad tube which hadbeen pointed for cold drawing;

FIG. 4 is a reproduction of a photograph taken at 0.74 diameterdepicting the pointed end of a cold drawn clad tube;

FIG. 5 is a reproduction of a photomicrograph taken at 1,000 diametersdepicting the microstructure ofa 50 percent nickel-5O percent chromiumalloy cladding; and

FIG. 6, which is partially broken away, is illustrative of a compositebillet produced in accordance with the invention.

In its broad aspects, the present invention provides a powder metallurgymethod for producing a wrought composite metal product comprising,providing a compacted self-supporting preform of uniform porosity frommetal powders having a composition such that, when substantially fullydensificd, the melting point thereof is at least about 2,300F.,assembling the preform in contact with a basis metal shape made of metalhaving a melting point of at least about 2,300F. and maintaining thepreform and basis metal in their assembled relationship while hotcoextruding the assembly through an extrusion die under protectiveconditions to produce a metallurgically bonded wrought composite metalproduct. The assembly may comprise a composite billet formed of a metalcore fitted within a preform comprising a sintered uniformly porousshell of metal powder which when hot consolidated has a melting point ofat least about 2,300F. The core and conforming shell may have anydesired cross-sectional configuration, for example, the cross sectionmay be a circle, an elipse, a square, a rectangle, and the like. Theassembly may alternatively comprise a composite billet formed ofasintered uniformly porous preform of metal powder fitted within asurrounding metal envelope.

By extruding the billet under protective conditions, a cladded metalproduct of good metallurgical quality is assured. Such conditions may beachieved by extruding in vacuum, or under a controlled atmospheresubstantially non-oxidizing to the billet, or by maintaining aprotective layer between the product being extruded and the extrusiondie, the latter method being advantageously preferred. Advantageously,the protective layer is a substantially nondecomposable inorganicmaterial capable of flowing under heat and pressure along the surface ofthe extrusion die and of providing lubricity during extrusion.

The inorganic material may be in the form of a dense deformable metalcoating covering the composite billet capable of flowing freely alongthe surface of the extrusion die during extrusion. The covering may beajacket or can of nickel, iron, or deformable nickel-base or iron-basealloys. A method of protection found particularly advantageous duringextrusion is to employ a deformable metal coating covering the billettogether with a layer of inorganic high temperature lubricant incontacting relationship with and between the deformable metal layer andthe die surface, the high temperature lubricant being characterized bythe property of viscous flow during extrusion. Examples of siliceoushigh temperature lubricants which may be used advantageously are mineralwool, basalt, slags, glass, and the like.

In its more preferred aspect, the present invention comprises forming,as a preform, the hollow shell from metal powder by pressing said powderabout a smooth former, removing the resulting preformed shell from theformer, sintering the shell under conditions nonoxidizing to the metalof the shell, placing within said shell a metal core having an outerconfiguration substantially matching the inner configuration of saidshell and hot extruding said core and said shell under protectiveconditions together to form a composite metal product. The step offorming the metal powder shell is highly important in relation to thepurposes of the present invention. It is desirable that the shell havean internal surface of as high a quality as possible and have uniformporosity, that is, uniform density. This objective is mostadvantageously attained by isostatically pressing about a rigid formeran initial powder mixture having a composition such that when hotconsolidated the melting point thereof is at least about 2,300F. Theformer employed may advantageously be made of polished metal such assteel, although other rigid metallic materials, plastic materials,glass, ceramics, etc., could be used. By this means, the inner face ofthe powder pressing conforms to the smooth face of the polished formerand is accordingly of high quality with regard to smoothness and freedomfrom mechanical defects.

In addition, by isostatically pressing the powder, a shell of uniformporosity or density is assured which enables it to be sintered with aminimum tendency towards cracking.

The powder metallurgy shell or clad-forming element may be produced tohave a composition selected from the group consisting of nickel,chromium, iron groupchromium alloys containing about 15 to 60 percentchromium, up to about 55 or 65 percent cobalt, with the balanceessentially up to about percent iron, up to about 85% nickel andmixtures thereof; and hotworkable nickel-base alloys containing lessthan 15 percent chromium, such as nickel-copper alloys containing about10 percent to about 45 percent copper and the balance essentiallynickel; nickel-molybdenum alloys containing up to about 40 percentmolybdenum and the balance essentially nickel; nickel-aluminum alloyscontaining up to about percent aluminum and the balance essentiallynickel; and other hot-workable nickel-base alloys.

The powders which may be employed for producing the shell may be readilyavailable commercial grades; for example, carbonyl nickel, iron andcobalt powders are eminently satisfactory. Electrolytic chromiumlikewise is satisfactory, although ferrochromium powders may be employedin interests of economy. Presently available ferrochromium powders tendto contain undesirably high amounts of foreign materials such as oxides,etc. Hydrogen-reduced mill scale may be employed as a source of ironpowder. Hydrogen-reduced nickel, cobalt and copper powders may also beemployed. In general, the initial powder mixture should be thoroughlyblended and preferably and advantageously prepared from powders havingparticle sizes not exceeding about 100 mesh in the longest dimension.Care should be taken to prevent segregation of the blended powders. Thepowders found particularly advantageous are those having particle sizesin the range of about 1 to about 40 microns, since the necessarydiffusion and alloying during sintering and extrusion are thenfacilitated. Alloyed and coated powders, e.g., nickel-coated chromiumpowders, may be employed to advantage. Advantageously, at least about 40or about 50 percent, by weight, of the powder in the initial mixture isa readily compressible powder of irregular shape having a particle sizenot exceeding 10 microns, e.g., carbonyl nickel powder, etc. Thepresence of such quantities of such powders in the mixture helps toprevent segregation of the powders and contributes importantly to thegreen strength of shells produced by isostatic compression at ambienttemperatures and at reasonable and economic pressures, e.g., pressuresranging up to about 50,000 pounds per square inch (p.s.i.). When morethan about 40 percent of the powders are spherical in shape, undesirablesegregation of powder mixtures occurs much more readily and the greenstrength of isostatically pressed shells suffers. Efforts to improvegreen strength of such powder mixtures through the use of pressingpressures exceeding 50,000 p.s.i., e.g., 100,000 p.s.i., or through theuse of elevated temperatures greatly complicate the shell-formingoperation, require the use of much more expensive equipment and are notuniformly successful. Such efforts also increase cost substantially.

The invention is particularly applicable to the production of extrudedarticles, e.g., tubes, having claddings made of nickel-chromium andnickel-chromiumiron alloys, e.g., alloys containing about to about 60%chromium, e.g., about or to about 50 percent chromium, up to aboutpercent iron and the balance essentially nickel. Minor amounts, e.g., upto about 20 percent or about 25 percent, of other ingredients may beemployed in these alloys as well as in the cladding metals describedhereinbefore. Such other ingredients, e.g., up to about 5 percentmanganese, up to about 12 or 15 percent columbium, up to about 15 or 20percent molybdenum, up to about 10 or 15 percent tungsten, up to about0.25 or 0.5 percent or even 1 percent carbon, up to about 2 or 3 percentsilicon, up to about 0.5 percent magnesium, up to about 5 percentaluminum, up to about 5 percent titanium, up to about 1 or 2 percentzirconium, etc., can be included in the initial powder mixture forspecial purposes. Carbon acts as a deoxidizer and can contribute tocarbide hardening. Preferably, silicon, magnesium, aluminum and titaniumpowders are coated with a metal such as nickel to minimize oxidation ofthese metals and/or interaction with other ingredients.

It is found that the chromium-containing metal powder mixtures can beisostatically pressed about a polished metal former and upon release ofthe pressure will spring back from the former thereby permitting easyremoval of the green powder pressing from the former even though theformer is made with parallel sides as in a cylinder. in the case ofpowder mixtures containing less than about 15 percent chromium, a slightdraft is advantageously provided on the polished metal former to permiteasy removal of the green powder shell therefrom since such powdermixtures, including chromium-free mixtures, do not exhibit the springback phenomenon to as great an extent. With such powder mixturesstraightness, smoothness and lack of ovality of the cylindrical formeris much more important than in the case of chromium-containing powdermixtures.

The core material, which is assembled with the shell before extrusion,may be made of an iron-group metal or an alloy containing a majorproportion of iron-group metal wherein the metal or alloy preferably,though not necessarily, has a melting point not exceeding that of ironand has a composition to satisfy the structural demands of the servicecontemplated for the final composite article. For purposes of economy,the core material may conveniently be a wrought, e.g., hot rolled,material such as a round. The exterior surface of the core material isconditioned to remove surface oxides and other imperfections thereon bymethods such as sand blasting, machining, pickling and the like. It isnot necessary that the exterior surface of the core be polished.

Examples of core metals which may be employed are those selected fromthe group consisting of iron; nickel; cobalt; low and medium alloysteels; and alloys containing about 5 to 30 percent chromium, up toabout 30 percent cobalt and the balance essentially up to about 90percent iron and up to about 85 percent nickel. The steels includecarbon steels and high strength low and medium alloy steels, stainlesssteels containing about 5 to about 30 percent chromium, up to about 30percent nickel and the balance essentially iron; and in addition nickel,and nickel alloys, such as nickel-chromium and nickel-chromium-ironalloys containing about 5 percent to about 30 percent chromium, up toabout percent iron and the balance essentially nickel, e.g., an alloycontaining about 30 percent to about 35 percent nickel, about 19 toabout 23 percent chromium, and the balance essentially iron and thelike. Age hardenable and dispersion-hardened"grades of nickel,nickelchromium and nickel-chromium-iron alloys can be employed as corematerials. Maraging steels may also be employed. An essentialcharacteristic of the core material is that it be readily bondable tothe shell material under the temperature and pressure conditionsemployed in the extrusion operation.

It is to be appreciated that the core material can be inserted in thepowder shell either before or after the sintering operation. Most greenpowder mixtures will shrink during sintering and accordingly dimensionalprovision is made between the outer dimensions of the core and the innerdimensions of the green shell to permit shrinkage to take place withoutcracking of the shell during Sintering. It is advantageous to insert thecore material within the shell when the shell is in the green, i.e.,unsintered, condition since the core material then provides somemechanical support for the shell during the sintering operation.Sintering is conducted in an atmosphere which is nonoxidizing to theshell material and at an elevated temperature, e.g., l,800F. to 2,400F.,having regard for the temperature requirements for sintering the shell.Some bonding of the shell material to the core material may, andadvantageously does, take place during the sintering operation. It is,however, not essential to extrusion that bonding between the core andthe shell be accomplished during the sintering. Sintering is desirablyconducted to such an extent that the sintered shell has sufficientstrength to be self-supporting and readily handled and to accomplishsubstantial diffusion and/or alloying in the sintered shell.

In preparing the composite material for extrusion, it is advantageouslyconvenient to encase the core and shell assembly within a deformablesupplemental metal can, or jacket, or coating which may be commerciallypure nickel, steel, etc., applied by welding sheet material, by dippingin molten metal, e.g., nickel, or by sintering a metal powder coatingapplied as a slurry. A slurry comprising fine metal powder, e.g.,carbonyl nickel powder, dispersed in a liquid medium, e.g., an organicmedium containing a binder, applied about the exterior of the sinteredshell assembled on the core may also be employed for protectivepurposes. At this point, sintered shells made from initial powdermixtures containing 15 percent or more chromium are not fully dense,being in excess of 70 percent and generally in excess of 80 percent oftheoretical density. The supplemental canning or coating procedure toapply a substantially dense deformable metal layer about the shellprotects it against oxidation during heating in mill furnaces prior toextrusion, while providing lubricity by flowing freely under heat andpressure during extrusion. If a steel or iron supplementary coating isemployed, heating for extrusion is conveniently accomplished underconditions which are essentially nonscaling to steel, e.g., in a saltbath furnace.

An advantageous and relatively inexpensive procedure for coatingsintered shells made of chromiumcontaining metal involves coating theshell with a slurry containing fine nickel powder, e.g., carbonyl nickelpowder having a particle size not exceeding about 10 microns, avolatilizable vehicle and a vehicle-soluble or dispersible binder. Thevehicle preferably is a volatile organic liquid such as kerosene, methylmethacrylate, etc. The slurry may contain, by weight, about 50 percentto about 80 percent of fine nickel powder, up to about 5 percent, e.g.,about 0.1 percent to about 5 percent, of a binder, with the remainderbeing the vehicle. An organic system comrising methyl methacrylate withan acrylic resin (Lucite") as a binder is satisfactory. The consistencyof the slurry is adjusted to the method of application, e.g., spraying,brushing, dipping, etc., by adjusting the contents of nickel powderand/or binder. After application of the slurry to the sintered shell, itis dried and sintered in a protective atmosphere, e.g., dry hydrogen, ata temperature sufficiently high, e.g., at least about 2,000F. and, moreadvantageously, about 2,200F. to sinter the nickel coating tosubstantially full density. Fine iron powder, e.g., carbonyl ironpowder, may be employed along with or in place of fine nickel powder inthe slurry. These procedures afford some protection against oxidation ofthe metals during heating for extrusion and, hence, are advantageous inthe case of sintered shells containing chromium. The supplementalcoating material also affords a lubricating effect during extrusion andsubsequent working operations. As stated hereinbefore, the coatingmaterial may be selected from the group consisting of nickel, iron, anddeformable nickel-base and iron-base alloys. In addition to thesupplemental coating, it is advantageous to employ a high temperaturelubricant, to further enhance lubricity during extrusion.

The coated assembly is then heated to the temperature required forextrusion, usually at least about 2,000F. In the case of a shellcomprising 50 percent nickel and 50 percent chromium, having a core madeof an alloy containing about 32 percent nickel, about 20 percentchromium, and the balance essentially iron, a heating temperature ofabout 2,200F. to about 2350F. for extrusion is satisfactory. The heatedassembly is then extruded to tubing in a conventional extrusion press.

During extrusion, metallurgical bonding of the shell material to thecore material is completed or accomplished and the shell material iscompacted to substantially percent density. Advantageously, an extrusionratio of about 10 to l to about 25 to l is employed. It is in theextrusion operation that the nature of the metals selected for use inthe core and in the shell become of particular importance. Thus, themetals in each portion of the composite material being extruded areselected to have melting points, in the fully alloyed condition, of atleast about 2,300F., e.g., at least about 2,350F., and preferably notexceeding the melting point of iron, i.e., about 2,800F., and morepreferably not exceeding about 2,650F. or even about 2,460F. (l,350C.).This special control, especially for a cladding alloy comprising about40 to 60 percent chromium and the balance essentially nickel,contemplated in accordance with the invention facilitates finalcompaction of shell material and bonding of the shell and core materialsin the extrusion operation. The socontrolled shell and core materialsextrude at similar rates and substantially uniform thickness of claddingis accomplished. It appears that bonding of the core and claddingmaterials is accomplished to a substantial extent during the upsettingof the composite extrusion billet in the extrusion chambers prior to thetime that metal is actually forced through the die. Such bonding enablescoextrusion of both materials having controlled melting points andprevents stripping away of the cladding from the core during extrusion.In addition, the aforementioned special control insures that'extrusionwill be conducted at a temperature substantially exceeding therecrystallization temperature of each material, a factor which furtherfacilitates bonding during extrusion. The special melting point controlalso assures that each portion of the composite will have similar hotworking and cold working characteristics.

With respect to a cladding alloy containing about l5 to 60 percentchromium and the balance essentially nickel, and particularly onecontaining about 35 to about 65 percent chromium, e.g., about 40 toabout 60 percent chromium, it will be noted according to thenickel-chromium binary diagram that alloys over such ranges ofcomposition have a lowest melting phase (a eutectic) which melts in theneighborhood of about 1345C. With alloys of the foregoing composition,the extrusion temperature should be at least about 85 percent of theabsolute melting point of the lowest melting phase and, moreadvantageously, from about 90 to 97 percent of the absolute meltingpoint of the lowest melting phase.

The extruded tube may then be further reduced by conventional hot and/orcold forming methods such as hot or cold drawing, tube reduction,swaging and the like, to provide dimensions required for specific uses.Advantageously, the extruded material is annealed before furtherprocessing. The thickness of the cladding in the final tubing is, ofcourse, controlled by the thickness of the initial shell and theproportion of said thickness to the transverse dimension of the corematerial. Claddings on the order of to 50 percent of the finalthickness, e.g., wall thickness of a tube, of the extrusion can readilybe provided.

In forming the shell from metal powders, isostatic pressing at ambienttemperatures is advantageously employed to insure uniform shell density.This method readily affords a means for producing a self-supportinggreen powder shell having sufficient strength to permit handling (e.g.,an apparent density of at least about 60 percent) at readily attainablepressures, for example, about 10,000 p.s.i. to about 40,000 p.s.i.,e.g., about 30,000 p.s.i., when a substantial proportion of a finereadily compressible powder is employed in the powder mixture. Theaccompanying FIG. 1 illustrates a mold assembly for pressing the powdershell. In FIG. 1, reference character 10 illustrates the smooth centralcore about which the powder mixture is to be pressed. This core may bemade of steel or any other conventional strong metal and itadvantageously is polished so as to provide a smooth interior surface onthe powder pressing. The assembly includes a metal container 11 which isplaced about the entire assembly and a bag 12 made of a plastic orelastomeric material such as rubber fitting within the container 11 andcompletely surrounding the core 10 and providing an annular spacetherebetween. Metal powder is packed in the annular space between bag 12and former 10 as designated at 13. Rubber spacer 14 is inserted at theupperportion and metal flange 15 is providd at the bottom of theannularly packed powder to maintain flat surfaces and square corners onthe powder during the compacting step and a rubber sealing disc 16 isemployed to close the annular space at the top of bag 12. The moldassembly as shown in FIG. 1 is subjected to hydrostatic pressure tocompress the powder and to form the green shell.

Composite metal products which may be produced in accordance with thepresent invention include those set forth in Table I.

9 50% nickel-50%chromium 15% chromium.7%

iron, balance nickel 10 15% chromium 7% iron, steel balance nickel 11nickel-30% copper steel 12 32% nickel, 20.5% steel chromium, balanceiron 13 60% chromium-40% nickel 15% chromium, 7%

iron. balance nickel 14 60% chromium-40% nickel steel It is alsopossible to produce the core material by powder metallurgy methodsinvolving pressing and sintering of powders having, or which are blendedto have, the desired composition. The core material may be produced as acasting, for example, by the methods described in US. Pat. Nos.2,882,568 and 2,973,563.

In order to give those skilled in the art a better understanding and/orappreciation of the advantages of the invention, the followingillustrative examples are given:

EXAMPLE I A powder mixture comprising about 50 percent, by weight, ofcarbonyl nickel powder having a particle size of about 5 microns andabout 50 percent, by weight, of minus 325 mesh electrolytic chromiumpowder was thoroughly blended. The powder was isostatically pressed at apressure of about 30,000 psi. about a smooth metal former using a moldassembly as illus trated in the accompanying FIG. 1. When the mold wasdisassembled, it was found surprisingly that the green pressednickel-chromium shell displayed sufficient spring-back to permit easyremoval of the green shell from the former. The pressed shell was in theform of a hollow cylinder having a diameter of about 8% inches, a wallthickness of about 0.625 inch, a length of about 30 inches, and wasself-supporting and had sufi'rcient green strength to permit handling. Ahot rolled extrusion billet made of an alloy containing about 32 percentnickel, about 20.5 percent chromium, and the balance essentially ironwas placed within the green shell. The billet had an outside diameter ofabout 7.150 inches, a length of about 29 inches, and was provided withan axial hole of about 2.25 inches in diameter along the length thereof.The outer surface of the extrusion billet was prepared by rough turning.The assembly was then heated at about 2,300F. in a hydrogen atmospherefor about 12 hours to sinter the shell. During the sintering operation,interdiffusion occurred between the nickel and chromium particles in theshell, the shell shrank, and some bonding occurred between the shell andthe core. The sintered shell was about percent dense. The assemblyincluding the sintered shell was cooled in hydrogen, was enclosed in awelded can made of commercial wrought nickel sheet about 0.187 inchthick, was heated to about 2,300F. for about 4 hours in a naturalgas-fired heating furnace, and was extruded with glass lubrication to atube having an outside diameter of about 2.75 inches, a wall thicknessof about 0.437 inch and a length of about 35 feet. The nickel-chromiumalloy cladding was about 25 percent of the wall thickness. When theextruded tube was examined no cracks or tears were noted in the claddingand the cladding thickness was uniform from the front to the back of thetube. The tube was annealed for 1% hours at 2,250 F. and was waterquenched. The annealed tube was pointed, and was cold drawn withoutdifficulty. It was found that the annealed cladding material had ahardness of about 92 Rockwell B, although a cast alloy of the samenominal composition had a hardness of 23 Rockwell C after extrusion anda similar anneal. Metallurgical examination of the extruded and the colddrawn material demonstrated that the nickel-chromium alloy cladding wassubstantially completely densified, although some oxide inclusionsapparently derived from oxides present in the initial chromium powderwere noted in the structure. The dense cladding was metallurgicallybonded to the core material. No checking of the coating was observed atthe cold drawn stage after removal of the nickel can material as isillustrated in the accompanying FIG. 2. The severe deformation inducedby the tube pointing operation did not cause any separation of the bondbetween the cladding and core metals as is illustrated by theaccompanying FIG. 3, which shows the structure of the transverse sectionof the tube in the pointed area. The outer surface of a portion of thetube, including the pointed area, is shown in FIG. 4. This exampledemonstrates that by proceeding in ac cordance with the invention, afinal wrought material having cladding containing 50 percent nickel and50 percent chromium may readily be produced. This cladding material isdifficultly hot workable by presently existing means when starting froma large cast ingot. The nickel-chromium-iron alloy was selected as thecore metal in this Example since it is characterized by good structuralstability upon long-time exposure at temperatures on the order of 900F.to l,500F. and performs well in contact with superheated steam insidesuperheater tubing. The 50 percent nickel-50 percent chromium claddingis practically immune to elevated temperature corrosion in the presenceof vanadium and sulfur compounds as is encountered on the outside faceof a superheater tubing. The clad tube accordingly is capable of almostindefinite life in superheaters fired with low-grade fuels such as coal,residual oil, etc.

EXAMPLE II An extruded tubing having cladding representing about 25percent of the wall thickness made of a 18 percent chromium-8 percentnickel stainless steel on a mild steel core is produced in accordancewith the procedures set forth in Example I employing an initial powdermixture comprising about I8 percent, by weight, of electrolytic chromiumpowder, about 8 percent, by weight, of carbonyl nickel powder, and thebalance iron powder derived from reduced mill scale to provide theinitial shell and employing a hot rolled mild steel core material withthe oxide surface removed by pickling. It was found that the stainlesssteel powder shell sprang back from the former after the isostaticpressing operation.

EXAMPLE III Carbonyl nickel powder having a particle size not exceedingabout microns is isostatically pressed to form a shell about 9 inches inoutside diameter and having a wall thickness of about 0.750 inch. Theformer employed is cylindrical with a draft of about 0. l inch from endto end. The nickel shell is removed from the former, is assembled upon arough turned hot rolled mild steel core having an axial hole 2.25 inchesin diameter therethrough, is sintered in a sulfur-free atmospherereducing to steel at about 2,200F. for about 10 hours, and is thenextruded directly to form a nickel clad steel tube having an outsidediameter of about 2.875 inches and a wall thickness of about 0.437 inchwith a nickel cladding representing about 25 percent of the wallthickness.

It is to be appreciated that the procedure made possible in accordancewith the present invention not only facilitates the production ofextruded composite materials having a cladding made of an alloy which isdiffi cult to hot work but also facilitates the production of compositeextruded articles made of readily workable materials with substantialeconomy. For example, in the production of stainless-clad mild steeltubing wherein a steel round is placed within a stainless steel tube andextruded therewith, it would normally be necessary to produce thestainless steel tube by conventional extrusion or piercing means.Substantial losses of metal are encountered in conventional processingfrom ingot to tubing form. Thus, losses of metal are encountered at theingot stage and at each intermediate stage down to the tubing.Furthermore, it is frequently necessary to provide overhauling of theinterior face of the stainless steel tubing in order to provide a soundmetal-- lurgical bond to the core material. Surface overhaul of theinterior face of a tube is expensive in terms of machine and labor costsand losses of metal. The procedure afforded in accordance with thepresent invention enables very high recovery of cladding metal in termsof weight of finished cladding material on the finished tube as comparedto the initial weight of the powders employed to produce the initialshell. This important commerical advantage enables a reduction in costof extruded clad materials as compared to other methods now available.

The method of isostatically pressing powders about a smooth former asdescribed hereinbefore is particularly advantageous in carrying out theinvention and solves many practical problems. Thus, if the powders aregravity packed or are isostatically pressed directly about the core,shrinkage cracking of the powder material during sintering is almostinevitable. Furthermore, efforts to control shrinkage by using coarserpowders, i.e., powders exceeding I00 mesh in size, and by controllingparticle size distribution are of little avail since the requiredinterdiffusion during sintering is then undesirably inhibited. Inaddition, any requirement for controlled powder particle size ordistribution severely limits the choice of powders amongst the fewgrades which are commercially available and would necessitate theproduction of special powders at high cost. The initial shell could beproduced by packing the powder mixture in a double walled mold made ofrubber or other plastic or elastomeric material having a core in sertedtherein during the packing operation, removing the core, andhydrostatically pressing the mold from both sides. However, this methodcomplicates the operation and usually does not provide as satisfactoryan interior surface on the shell. Again, the shell could be pressedagainst a mold placed about the exterior thereof, with a rubber moldagainst the shell inner face. This procedure also requires a core duringthe mold filling step and introduces processing difficulties.

It is to be appreciated that multiple claddings can be provided inaccordance with the invention by nesting together two or more sinteredand/or pressed shells with an inner core and hot extruding the assembly.

As illustrative of one embodiment of the composite extrusion billetprovided by the invention, reference is made to FIG. 6 which shows,partially broken away, a can 17 fabricated, for example, from nickelsheet, having a cover 18 welded thereto, the can having confined withinit a metal core 19 of, for example, a nickelchromium-iron alloycontaining approximately 20 percent chromium, approximately 32 percentnickel and the balance essentially iron. The core is shown having a hole20 passing axially through it, the core being fitted snugly within ahollow porous shell 21 of sintered cladding metal of, for example, anickel-chromium alloy containing approximately 50 percent chromium andthe balance essentially nickel.

As previously noted, oxide inclusions flowing from oxides present in theinitial chromium powder were found in the 50 percent nickel-O percentchromium alloy cladding produced as described in Example I.

These inclusions can be removed, with improvement in ductility anddensity of the cladding by employing the procedure described in U.S.Pat. No. 3,357,826 in the names of H.H. Honaker and J.C. Judd, in whichsmall additions of carbon and of a spacing agent such as ammoniumchloride are employed in the initial powder mixture whereby chromiumoxide is reduced during sintering in hydrogen.

A special feature of the invention as applied to composite extrudedproducts having a cladding of a 50 percent nickel-50 percent chromiumalloy on a base made of steel, nickel, nickel-chromium alloys, stainlesssteel, etc., is that the product can be bent to shape and can be coldworked. Annealing of the extruded product at a temperature on the orderof 2,200F., preferably in a hydrogen atmosphere, is desirable beforecold working, although the extruded product can be cold drawn withoutannealing. Thus, a slice from an extruded tube having a cladding of 50percent nickel-50 percent chromium alloy on a base of a 32 percentnickel, 20 percent chromium, balance iron alloy which had been annealedat 2,200F. for 1 hour and subjected to two consecutive cold drawingoperations involving 9 percent reductions in each pass was subjected tocold rolling. The slice was cold rolled to reduce the thickness thereof48 percent with little edge cracking. When annealed at 2,200F. for onehour in hydrogen and cooled in hydrogen, the slice could be bent about a1 inch diameter pin without fracture. The cladding and the base metaleach was reduced proportionally in terms of the original thicknessthereof. In contrast to this result, it appears that conventionallyproduced 50 percent nickel-50 percent chromium alloy is extremelydifficult to cold work and it is not available commercially at thepresent time in cold worked mill forms. It appears that the finemicrostructure which characterizes alloys containing about 40 to about60 percent chromium, with the balance essentially nickel when producedas a cladding material contributes importantly to the working qualitiesthereof. This structure is depicted in FIG. 5 which shows theas-extruded structure of a 50% nickel-50 percent chromium alloy claddingon an extruded tube produced in accordance with the procedure describedin Example I. The darker etching constituent is believed to be alphaphase.

While the invention has been described hreinbefore principally inconnection with the production of clad tubing, it is to be appreciatedthat composite clad mill forms including sheet and strip can also beproduced. In the production of flat mill forms, it is still advantageousto prepare an extrusion billet as described hereinbefore and to hotextrude the billet to a rectangular form. The extruded section can thenbe cut in lengths and cross-rolled hot to sheet or can be hot rolled tostrip. The hot extrusion step is necessary to accomplish compaction andbonding of clad material comprising about 40 percent to about 60 percentchromium, e.g., about 50 percent chromium, with the balance essentiallynickel. Single clad flat mill forms can be prepared by extruding acomposite billet in which the powder shell is split longitudinally tocover one half of the surface of a cylindrical basis metal billet whilethe remainder of the billet surface is covered with a longitudinallysplit shell of matching dimension made of metal having a composition thesame as or different from the basis metal billet. In this case, care istaken to place the composite billet in the extrusion press such that thecladding metal will be located on one surface of the extrudedrectangular section form. The production of extruded rectangular shapesis facilitated by using a rectangular shaped billet, using a rectangularextrusion press container, by making both the cladding metal preformportion and the basis metal portion of the composite billet from metalpowders, and by maintaining the cladding metal preform portion incontact with the composite powder billet during extrustion.

The concepts of the invention may also be applied to the production ofcomposite tubes having a corrosionresistant alloy cladding, e.g., theaforementioned nickel-chromium alloys containing about 15 percent toabout 60 percent chromium, on the inner face of the tube and with theouter face of the tube being made of a metal or alloy, e.g., a steel, anickel-chromium iron alloy, etc., having lesser corrosion resistance buthaving other desired metallurgical properties. In such a case, a mixtureof metal powders blended to the cladding material composition is formedas a tube of substantially uniform porosity and of appropriatedimension, is sintered in a protective atmosphere either before or afterinsertion into the bore of a hollow cylindrical shape of appropriatedimension for an extrusion billet and made of a composition such asthose described hereinbefore in terms of a core material in the case ofa composite tube having an outer cladding, is then covered on the innerface and ends with a thin covering of a metal such as nickel or mildsteel and the assembly is then heated and co-extruded hot to form acomposite, wrought, extruded tube.

The pressed tube may be prepared by isostatic pressing of a metal powdermixture about a solid former in the manner described in conjunction withFIG. 1. Altematively, a nickel or iron tube may be used in place of thesolid former with the tube being left in place during subsequentoperations to protect the pressed powder mixture during heating forextrusion. When this is done, the diameter of the tube will usually bereduced during isostatic pressing. The tube may be filled with granularmaterial such as sand to restrict the amount of reduction. The outerdiameter of the sintered tube may be sized by machining or grindingprior to assembly in the extrusion billet.

As a further alternative, the powder mixture of corrosion resistantcomposition may be pressed isostatically against the inner face of theheavy-walled, hollow cylindrical shape of metal with which the pressedpowder mixture, after sintering, is to form the composite extrusionbillet. Such a procedure offers the advantage that, during thesubsequent sintering operation, bonding occurs between the sinteredmetal powder and the mating surface of less corrosion-resistant alloy ormetal. The

procedure is readily accomplished by placing concentrically within thebore of a heavy-walled, hollow cylindrical shape having an outerdiameter and length desired for the composite extrusion billet a metaltube having a number of holes in the wall thereof, which tube issurrounded by a tube or bag of rubber or other elastomeric material,leaving an annular space between the rubber tube and the inner wall ofthe hollow cylindrical shape, which annular shape extends substantiallyalong the entire length of the shape. The annular space is then filledwith a metal powder mixture, for example, a 50 percent nickel-50 percentchromium powder mixture, and the ends of the powder annulus are sealed,for example, with rubber spacers to separate compaction fluid from thepowder. The entire assembly is placed in a pressure vessel andcompressed by hydraulic pressure admitted through the holes in thecenter metal tube and transmitted the full length of the rubber tube.Upon removal of the assembly from the pressure vessel and removal of thecenter metal tube, the rubber tube, and the spacers, it is found thatthe powder is compressed against the inner wall of the heavy-walled,hollow cylindrical shape. The assembly may than be subjected to asintering operation in a protective atmosphere as describedhereinbefore, the exposed surfaces of the sintered powder may then becovered with a thin metal covering and the resulting assembly may thenbe heated to extrusion temperature and hot extruded. It is an advantagefrom the standpoint of microstructural cleanliness of the sinteredpowder portion of the composite billet that at least one longitudinal(major) surface thereof be exposed to the protective atmosphere, e.g.,hydrogen during the sintering operation.

As is apparent, the invention provides a method for producing wroughtcomposite metal products from a wide range of cladding metals, includingmetals normally difficult to produce by conventional claddingtechniques. In its preferred aspects the invention comprises forming anisostatically pressed self-supporting preform of uniform porosity frommetal powders having a composition such that, when hot consolidated, themelting point thereof is at least about 2,300F., sintering the shape ina nonoxidizing atmosphere, assembling the shape in faying relationshipwith a basis metal shape having a composition with a melting point of atleast about 2,300F., and then maintaining the shapes in fayingrelationship while hot coextruding said shapes through an extrusion dieto produce a metallurgically bonded metal article.

In protecting the metal powder Shape while in faying relationship withthe basis metal during extrusion, it is advantageous to employ aninorganic material such as an easily deformable metal layer against thecompacted metal powder layer, capable of flowing under heat and pressurealong the surface of the extrusion die. Preferably, the protection maycomprise a plural layer structure formed of a protective deformablemetal layer, e.g., nickel or iron, or the like, covering the compactedmetal powder layer with a layer of inorganic material in contactingrelationship with and between the deformable metal layer and the diesurface capable of viscous flow along the die surface, whereby toprovide additional lubricity during extrusion to ensure production of ametal cladding of good metallurgical quality.

The invention also provides a composite extrusion billet comprising abasis metal of melting point at least about 2,300F., a consolidatedself-supporting uniformly porous preform of metal held in fayingrelationship to the surface of the basis metal, the composition of themetal being such as to have a melting point of at least about 2,300F.and advantageously a protective metal coating covering the assembly, themetal coating being capable of flowing under heat and pressure andproviding lubricity during extrusion. In one embodiment of theinvention, the basis metal may be a metal core and the compacteduniformly porous preform may be a sintered hollow shell fitted over themetal core, the assembled core and shell being in turn covered by aprotective deformable metal coating, such as nickel, iron, and the like.

Cladded extruded products produced in accordance with the invention havea multiplicity of uses wherein articles having corrosion resistantsurfaces upon basis metals having structural strength and utility arerequired. Thus, power plant tubing is an area of immediate interestsince a composite tubing having a surface made of an alloy containingabout 40 to about percent chromium, e.g., about 50 percent chromium,with the balance essentially nickel and a basis of a nickelchromium ornickel-chromium-iron structural alloy is useful in superheater tubingand a composite tubing having a chromium-nickel alloy surface and astructural steel base is useful in cold wall" power plant applications.Cladded materials are also useful in chemical equipment handlingcorrosive materials. Thus, heating coils, evaporators, autoclaves andother reactors, etc., can be constructed in whole or in part ofmaterials provided in accordance with the invention. Cladded articlesprovided in accordance with the invention can be welded usingconventional welding techniques using filler materials matching thecomposition of each alloy component of the article. Thus, welds havebeen made in a cladding of 50 percent nickel-5O percent chromium alloyusing the inert-gas shielded tungsten-arc procedure with matchingcomposition filler metal.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. A method for producing a wrought composite metal tube having a powdermetallurgy cladding on the internal surface thereof which comprisesisostatically pressing metal powders having a composition such that,when substantially fully densified, the melting point thereof is atleast about 2,300F. and which are selected from the group consisting ofnickel, chromium, hot-workable nickel-base alloys containing less than15 percent chromium and hot workable iron-groupchromium alloyscontaining about 15 to 60 percent chromium, up to about percent cobaltwith the balance essentially up to about percent iron, up to about 85percent nickel and mixtures thereof to form a compacted self-supportingpreform of uniform porosity and tubular shape, assembling said preformin contact with the interior surface of a hollow, substantiallycylindrical basis metal shape made of metal having a melting point of atleast about 2,300F. and having a composition selected from the groupconsisting of iron,

nickel, cobalt, low and medium alloy steels, and an alloy containingabout to 30 percent chromium, up to about 30 percent cobalt, and thebalance essentially up to about 90 percent iron and up to about 85percent nickel, sintering said preform in a protective atmospherecovering at least the sintered metal portion of said assembly with aprotective deformable metal coating capable of flowing under heat andpressure and providing lubricity during extrusion and maintaining saidpreform and basis metal in said assembled relationship along with saidcoating while hot coextruding them together through an extrusion dieunder protective conditions to produce a metallurgically bonded, wroughtcomposite metal tube.

2. The method according to claim 4 wherein said preform is sinteredprior to said assembly.

3. The method according to claim 4 wherein said metal powders areisostatically pressed against the interior surface of said hollow,substantially cylindrical basis metal shape and are thereafter sintered.

4. A method for producing a wrought composite metal tube having a powdermetallurgy cladding on the internal surface thereof which comprises,isostatically pressing metal powders having a composition such that,when substantially fully densified, the melting point thereof is atleast about 2,300F. to form a compacted self-supporting preform ofsubstantially uniform porosity and tubular shape, sintering said preformin a protective atmosphere, assembling said preform in contact with theinterior surface of a hollow, substantially cylindrical fully densebasis metal shape made of metal having a melting point of at least about2,300F. and mairataining said preform and basis metal in said assembledrelationship while coextruding them together at a temperature of atleast about 2,000F. through an extrusion die under protective conditionsto produce a metallurgically bonded wrought composite metal tube havinga thickness ratio of cladding to basis metal approximating the thicknessratio of the preform to the basis metal.

2. The method according to claim 4 wherein said preform is sinteredprior to said assembly.
 3. The method according to claim 4 wherein saidmetal powders are isostatically pressed against the interior surface ofsaid hollow, substantially cylindrical basis metal shape and arethereafter sintered.
 4. A method for producing a wrought composite metaltube having a powder metallurgy cladding on the internal surface thereofwhich comprises, isostatically pressing metal powders having acomposition such that, when substantially fully densified, the meltingpoint thereof is at least about 2,300*F. to form a compactedself-supporting preform of suBstantially uniform porosity and tubularshape, sintering said preform in a protective atmosphere, assemblingsaid preform in contact with the interior surface of a hollow,substantially cylindrical fully dense basis metal shape made of metalhaving a melting point of at least about 2,300*F. and maintaining saidpreform and basis metal in said assembled relationship while coextrudingthem together at a temperature of at least about 2,000*F. through anextrusion die under protective conditions to produce a metallurgicallybonded wrought composite metal tube having a thickness ratio of claddingto basis metal approximating the thickness ratio of the preform to thebasis metal.